Project Summary The plant hormone auxin is involved in virtually all aspects of plant growth and development. Over the years we and others have demonstrated that auxin acts by stimulating the degradation of a family of transcriptional repressors called the Aux/IAA proteins, through the action of the ubiquitin protein ligase SCFTIR1/AFB. Although this basic pathway is well established, we still know very little about how auxin regulates plant growth and development. Strikingly, auxin-regulated genes differ dramatically between cell types and organs, consistent with the diverse roles of the hormone. The basis for this specificity is unknown. Similarly, the activities of the Aux/IAA and ARF proteins are poorly characterized and the gene regulatory networks (GRNs) that mediate various auxin-regulated growth processes have not been defined. Finally, we know little about how auxin signaling is integrated with other environmental and genetic signaling pathways. Recently we discovered that one member of the TIR1/AFB family of auxin co-receptors, AFB1, has a key role in a non-genomic rapid auxin response in the root. This was a surprising result because the TIR1/AFB proteins are known to function in transcriptional regulation. We also demonstrated that AFB1 is highly enriched in the cytoplasm and propose that it is this feature that leads to its specialized role in the rapid response. In this proposal, we request funds to purchase a Keyence BZX 810 fluorescent microscope. This instrument is ideally suited to assess the growth of Arabidopsis seedlings in real time. We have already demonstrated its utility in studies of the role of auxin in rapid changes in root growth using an instrument at the Salk Institute. We expect that the microscope will allow us to rapidly make progress in our understanding of a novel and surprising function of the AFB1 protein